1. Field of the Invention
The present invention relates to the switching of power to a load, and more particularly, to solid state relays and methods for switching power to a load very quickly.
2. Background Art
Relays are electrically controlled switching devices. Electromechanical relays provide switching with a mechanical device typically controlled by an electromagnet. Solid state relays use semiconductors to provide switching. Solid state relays are frequently used to control a power supply to a load. Typically, a low voltage control signal is applied to a semiconductor switch to enable or disable a higher voltage power supply. Because the solid state relay controls a high voltage, high current power supply to a load, electrical isolation between the control circuitry and the load power is required. Transformers may be used to provide isolation between the low power circuitry and the high power circuitry in solid state relays. However, transformers should not be used in high shock environments, where there is a potential for mechanical shock or vibration. Photodiode opto-isolators may be used to control gates in a power switching stage; however, photodiode opto-isolators provide very little power and take a long time to charge the gates to switch the power supply. Alternatively, in order to generate a high enough voltage to enable a semiconductor switch, many photodiodes may be stacked in series.
TRIAC (TRIode for Alternating Current) or SCR (silicon controlled rectifier) based solid state relays remain on until the load current drops below a certain threshold, which occurs every ½ cycle in an AC powered system. Thus, TRIAC and SCR based solid state relays have a maximum turn off time of a ½ cycle, which may be more than 8 ms (milliseconds) at 60 Hz, to disconnect power to a load. In addition to the ½ cycle maximum turn-off time, TRIAC and SCR based solid state relays can have issues when powering a low power factor load. With a low power factor load, the solid state relays will attempt to turn off when the current approaches zero, which will cause a fast voltage transient (dv/dt) because the voltage will not be at zero volts. This can cause the solid state relays to latch up into the on-state. A snubber circuit can be used to slow down this voltage transient to minimize this issue.